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Damped spin-wave excitation in the itinerant antiferromagnet -FeMn$_{0.3}
Abstract
The collective spin-wave excitation in the antiferromagnetic state of -FeMn was investigated using the inelastic neutron scattering technique. The spin excitation remains isotropic up to the high excitation energy, meV. The excitation gradually becomes broad and damped above 40 meV. The damping parameter reaches 110(16) meV at meV, which is much larger than that for other metallic compounds, e.g., CaFeAs (24 meV) and LaSrMnO ( meV). In addition, the spin-wave dispersion shows a deviation from the relation above 40 meV. The group velocity above this energy increases to 470(40) meV{\AA}, which is higher than that at the low energies, meV{\AA}. These results suggest that the spin-wave excitation merges with the continuum of the individual particle-hole excitations at 40 meV.
... 5(f) and 5(h)], the INS spectra are heavily damped for both the single-Q and triple-Q phases compared to the simulation results. While this makes a more detailed comparison with the LLD simulations challenging, such deviations from the spin-wave theory's predictions, which are based on a fully localized picture of the magnetic moments, are commonly observed for high-energy excitations in metallic antiferromagnets [49][50][51][52][53]. This phenomenon is associated with the prevalence of the Stoner continuum in their energy-momentum space, whose influence generally increases with energy transfer [49][50][51]54]. ...
... While this makes a more detailed comparison with the LLD simulations challenging, such deviations from the spin-wave theory's predictions, which are based on a fully localized picture of the magnetic moments, are commonly observed for high-energy excitations in metallic antiferromagnets [49][50][51][52][53]. This phenomenon is associated with the prevalence of the Stoner continuum in their energy-momentum space, whose influence generally increases with energy transfer [49][50][51]54]. This metallic character may also be responsible for the reduced ordered moment of Co 1/3 TaS 2 in the triple-Q phase: µ s = 1.3µ ...
Multi-Q magnetic structures on triangular lattices, with their two-dimensional topological spin texture, have attracted significant interest. However, unambiguously confirming their formation by excluding the presence of three equally-populated single-Q domains remains challenging. In the metallic triangular lattice antiferromagnet Co1/3TaS2, two magnetic ground states have been suggested at different temperature ranges, with the low-temperature phase being a triple-Q structure corresponding to the highest-density Skyrmion lattice. Using inelastic neutron scattering (INS) and advanced spin dynamics simulations, we demonstrate a clear distinction in the excitation spectra between the single-Q and triple-Q phases of Co1/3TaS2 and, more generally, a triangular lattice. First, we refined the spin Hamiltonian by fitting the excitation spectra measured in its paramagnetic phase, allowing us to develop an unbiased model independent of magnetic ordering. Second, we observed that the two magnetically ordered phases in Co1/3TaS2 exhibit markedly different behaviors in their long-wavelength Goldstone modes. Our spin model, derived from the paramagnetic phase, confirms that these behaviors originate from the single-Q and triple-Q nature of the respective ordered phases, providing unequivocal evidence of the single-Q to triple-Q phase transition in Co1/3TaS2. Importantly, we propose that the observed contrast in the long-wavelength spin dynamics between the single-Q and triple-Q orderings is universal, offering a potentially unique way to distinguish a generic triple-Q ordering on a triangular lattice from its multi-domain single-Q counterparts. We describe its applicability with examples of similar hexagonal systems forming potential triple-Q orderings. (For the full abstract, please refer to the manuscript)
... However, the scattering intensity and dispersion deviate from the calculation at the zone boundary and well-defined modes are essentially absent above 200 meV in the measurements. This discrepancy in the scattering intensity is ascribed to interactions with the Stoner continuum [36][37][38][39], and indicates the energy scale of the Stoner excitations. Due to the large number of electronic bands in FeSn, it is challenging to make direct comparisons to the magnetic spectra measured here. ...
The kagome lattice is a fertile platform to explore topological excitations with both Fermi-Dirac and Bose-Einstein statistics. While relativistic Dirac fermions and flat bands have been discovered in the electronic structure of kagome metals, the spin excitations have received less attention. Here, we report inelastic neutron scattering studies of the prototypical kagome magnetic metal FeSn. The spectra display well-defined spin waves extending to 120 meV. Above this energy, the spin waves become progressively broadened, reflecting interactions with the Stoner continuum. Using linear spin-wave theory, we determine an effective spin Hamiltonian that reproduces the measured dispersion. This analysis indicates that the Dirac magnon at the K point remarkably occurs on the brink of a region where well-defined spin waves become unobservable. Our results emphasize the influential role of itinerant carriers on the topological spin excitations of metallic kagome magnets.
... However, the scattering intensity and dispersion deviate from the calculation at the zone boundary and well-defined modes are essentially absent above 200 meV in the measurements. This discrepancy in the scattering intensity is ascribed to interactions with the Stoner continuum [35][36][37][38], and indicates the energy scale of the Stoner excitations. Due to the large number of electronic bands in FeSn, it is challenging to make direct comparisons to the magnetic spectra measured here. ...
The kagome lattice is a fertile platform to explore topological excitations with both Fermi-Dirac and Bose-Einstein statistics. While relativistic Dirac Fermions and flat-bands have been discovered in the electronic structure of kagome metals, the spin excitations have received less attention. Here we report inelastic neutron scattering studies of the prototypical kagome magnetic metal FeSn. The spectra display well-defined spin waves extending up to 120 meV. Above this energy, the spin waves become progressively broadened, reflecting interactions with the Stoner continuum. Using linear spin wave theory, we determine an effective spin Hamiltonian that reproduces the measured dispersion. This analysis indicates that the Dirac magnon at the K-point remarkably occurs on the brink of a region where well-defined spin waves become unobservable. Our results emphasize the influential role of itinerant carriers on the topological spin excitations of metallic kagome magnets.
... Another is the decay of a magnon into an electron-hole pair (the Stoner continuum), which is common in metallic antiferromagnets. The former is known to exhibit unique q-dependence of Γ(q, E q ) according to the previous studies on insulating TLAFs [2,33], while the latter commonly leads to rather monotonic and straightforward q-dependence of Γ(q, E q ) [34][35][36]. In this respect, the magnon decay in CrB 2 is more likely to stem from the two-magnon effect, with the Stoner continuum effect becoming significant at the higher q points shown in Fig. 4. ...
Non-collinear magnetic order arises for various reasons in several magnetic systems and exhibits interesting spin dynamics. Despite its ubiquitous presence, little is known of how magnons, otherwise stable quasiparticles, decay in these systems, particularly in metallic magnets. Using inelastic neutron scattering, we examine the magnetic excitation spectra in a metallic non-collinear antiferromagnet CrB, in which Cr atoms form a triangular lattice and display incommensurate magnetic order. Our data show intrinsic magnon damping and continuum-like excitations that cannot be explained by linear spin wave theory. The intrinsic magnon linewidth shows very unusual momentum dependence, which our analysis shows to originate from the combination of two-magnon decay and the Stoner continuum. By comparing the theoretical predictions with the experiments, we identify where in the momentum and energy space one of the two factors becomes more dominant. Our work constitutes a rare comprehensive study of the spin dynamics in metallic non-collinear antiferromagnets. It reveals, for the first time, definite experimental evidence of the higher-order effects in metallic antiferromagnets.
Collective spin excitations in magnetically ordered crystals, called magnons or spin waves, can serve as carriers in novel spintronic devices with ultralow energy consumption. The generation of well-detectable spin flows requires long lifetimes of high-frequency magnons. In general, the lifetime of spin waves in a metal is substantially reduced due to a strong coupling of magnons to the Stoner continuum. This makes metals unattractive for use as components for magnonic devices. Here, we present the metallic antiferromagnet CeCo2P2, which exhibits long-living magnons even in the terahertz (THz) regime. For CeCo2P2, our first-principle calculations predict a suppression of low-energy spin-flip Stoner excitations, which is verified by resonant inelastic X-ray scattering measurements. By comparison to the isostructural compound LaCo2P2, we show how small structural changes can dramatically alter the electronic structure around the Fermi level leading to the classical picture of the strongly damped magnons intrinsic to metallic systems. Our results not only demonstrate that long-lived magnons in the THz regime can exist in bulk metallic systems, but they also open a path for an efficient search for metallic magnetic systems in which undamped THz magnons can be excited.
The investigations of the interconnection between micro- and macroscopic properties of materials hosting noncollinear antiferromagnetic ground states are challenging. These forefront studies are crucial for unraveling the underlying mechanisms at play, which may prove beneficial in designing cutting-edge multifunctional materials for future applications. In this context, Mn5Si3 has regained scientific interest since it displays an unusual and complex ground state, which is considered to be the origin of the anomalous transport and thermodynamic properties that it exhibits. Here, we report the magnetic exchange couplings of the noncollinear antiferromagnetic phase of Mn5Si3 using inelastic neutron scattering measurements and density functional theory calculations. We determine the ground-state spin configuration and compute its magnon dispersion relations which are in good agreement with the ones obtained experimentally. Furthermore, we investigate the evolution of the spin texture under the application of an external magnetic field to demonstrate theoretically the multiple field-induced phase transitions that Mn5Si3 undergoes. Finally, we model the stability of some of the material's magnetic moments under a magnetic field and we find that very susceptible magnetic moments in a frustrated arrangement can be tuned by the field.
The investigations of the interconnection between micro- and macroscopic properties of materials hosting noncollinear antiferromagnetic ground states are challenging. These forefront studies are crucial for unraveling the underlying mechanisms at play, which may prove beneficial in designing cutting edge multifunctional materials for future applications. In this context, Mn5Si3 has regained scientific interest since it displays an unusual and complex ground state, which is considered to be the origin of the anomalous transport and thermodynamic properties that it exhibits. Here, we report the magnetic exchange couplings of the noncollinear antiferromagnetic phase of Mn5Si3 using inelastic neutron scattering measurements and density functional theory calculations. We determine the ground-state spin configuration and compute its magnon dispersion relations which are in good agreement with the ones obtained experimentally. Furthermore, we investigate the evolution of the spin texture under the application of an external magnetic field to demonstrate theoretically the multiple field-induced phase transitions that Mn5Si3 undergoes. Finally, we model the stability of some of the material's magnetic moments under a magnetic field and we find that very susceptible magnetic moments in a frustrated arrangement can be tuned by the field.
By combining two independent approaches, inelastic neutron-scattering measurements and density-functional-theory calculations, we study the spin waves in the collinear antiferromagnetic phase (AFM2) of Mn5Si3. We obtain its magnetic ground-state properties and electronic structure. This study allowed us to determine the dominant magnetic exchange interactions and magnetocrystalline anisotropy in the AFM2 phase of Mn5Si3. Moreover, the evolution of the spin excitation spectrum is investigated under the influence of an external magnetic field perpendicular to the anisotropy easy axis. The low-energy magnon modes show a different magnetic field dependence, which is a direct consequence of their different precessional nature. Finally, possible effects related to the Dzyaloshinskii-Moriya interaction are also considered.
By combining two independent approaches, inelastic neutron scattering measurements and density functional theory calculations, we study the spin-waves in the high-temperature collinear antiferromagnetic phase (AFM2) of MnSi. We obtain its magnetic ground-state properties and electronic structure. This study allowed us to determine the dominant magnetic exchange interactions and magnetocrystalline anisotropy in the AFM2 phase of MnSi. Moreover, the evolution of the spin excitation spectrum is investigated under the influence of an external magnetic field perpendicular to the anisotropy easy-axis. The low energy magnon modes show a different magnetic field dependence which is a direct consequence of their different precessional nature. Finally, possible effects related to the Dzyaloshinskii-Moriya interaction are also considered.
Noncollinear magnetic order arises for various reasons in several magnetic systems and exhibits interesting spin dynamics. Despite its ubiquitous presence, little is known of how magnons, otherwise stable quasiparticles, decay in these systems, particularly in metallic magnets. Using inelastic neutron scattering, we examine the magnetic excitation spectra in a metallic noncollinear antiferromagnet CrB2, in which Cr atoms form a triangular lattice and display incommensurate magnetic order. Our data show intrinsic magnon damping and continuumlike excitations that cannot be explained by linear spin wave theory. The intrinsic magnon linewidth Γ(q,Eq) shows very unusual momentum dependence, which our analysis shows to originate from the combination of two-magnon decay and the Stoner continuum. By comparing the theoretical predictions with the experiments, we identify where in the momentum and energy space one of the two factors becomes more dominant. Our work constitutes a rare comprehensive study of the spin dynamics in metallic noncollinear antiferromagnets. It reveals, for the first time, definite experimental evidence of the higher-order effects in metallic antiferromagnets.
The High Resolution Chopper Spectrometer (HRC) provides opportunities for dynamical studies of materials in a wide energy-momentum space with high resolution in the world standards. In particular, the option of neutron Brillouin scattering (NBS) makes the HRC different from other chopper spectrometers. Coherent excitations in non-single-crystal samples can be observed with NBS. On the HRC, NBS experiments were made feasible and some results were obtained.
We present the concept of a novel time-focusing technique for neutron spectrometers, which allows us to disentangle time-focusing from beam divergence. The core of this approach is a double rotating-crystal monochromator that can be used to extract a larger wavelength band from a white beam, thus providing a higher flux at the sample compared to standard time-of-flight instruments, yet preserving energy resolution and beam collimation. The performances of a spectrometer based on this approach are quantitatively discussed in terms of possible incident wavelengths, flux at the sample, and (Q, E)-resolution. Analytical estimates suggest flux gains of about one order of magnitude at comparable resolutions in comparison to conventional time-of-flight spectrometers. Moreover, the double monochromator configuration natively shifts the sample away from the source line-of-sight, thus significantly improving the signal-to-noise ratio. The latter, in combination with a system that does not increase the beam divergence, brings the further advantage of a cleaner access to the low-Q region, which is recognized to be of fundamental interest for magnetism and for disordered materials, from glasses to biological systems.
Seven time-of-flight quasielastic/inelastic neutron scattering instruments are installed on six beamlines in the Materials and Life Science Experimental Facility (MLF) at the Japan Proton Accelerator Research Complex (J-PARC). Four of these instruments are chopper-type direct-geometry spectrometers, one is a near-backscattering indirect-geometry spectrometer, and the other two are spin-echo-type spectrometers. This paper reviews the characteristic features, recent scientific outcomes, and progress in development of MLF spectrometers.
Weyl fermions that emerge at band crossings in momentum space caused by the spin-orbit interaction act as magnetic monopoles of the Berry curvature and contribute to a variety of novel transport phenomena such as anomalous Hall effect and magnetoresistance. However, their roles in other physical properties remain mostly unexplored. Here, we provide evidence by neutron Brillouin scattering that the spin dynamics of the metallic ferromagnet SrRuO3 in the very low energy range of milli-electron volts is closely relevant to Weyl fermions near Fermi energy. Although the observed spin wave dispersion is well described by the quadratic momentum dependence, the temperature dependence of the spin wave gap shows a nonmonotonous behaviour, which can be related to that of the anomalous Hall conductivity. This shows that the spin dynamics directly reflects the crucial role of Weyl fermions in the metallic ferromagnet.
We present a general study of the magnetic excitations within a weak-coupling
five-orbital model relevant to itinerant iron pnictides. As a function of
enhanced electronic correlations, the spin excitations in the symmetry broken
spin-density wave phase evolve from broad low-energy modes in the limit of weak
interactions to sharply dispersing spin wave prevailing to higher energies at
larger interaction strengths. We show how the resulting spin response at high
energies depends qualitatively on the magnitude of the interactions. We also
calculate the magnetic excitations in the nematic phase by including an orbital
splitting, and find a pronounced C_2 symmetric excitation spectrum right above
the transition to long-range magnetic order. Finally, we discuss the C_2 versus
C_4 symmetry of the spin excitations as a function of energy for both the
nematic and the spin-density wave phase.
National user facilities such as the NIST Center for Neutron Research (NCNR)
require a significant base of software to treat the data produced by their specialized
measurement instruments. There is no universally accepted and used data
treatment package for the reduction, visualization, and analysis of inelastic
neutron scattering data. However, we believe that the software development
approach adopted at the NCNR has some key characteristics that have resulted in a
successful software package called DAVE (the Data Analysis and Visualization
Environment). It is developed using a high level scientific programming language,
and it has been widely adopted in the United States and abroad. In this paper we
describe the development approach, elements of the DAVE software suite, its
usage and impact, and future directions and opportunities for development.
The recent twenty years have witnessed significant progress in the basic understanding of the dynamical magnetic properties in metallic and strongly correlated materials, mainly due to the progress in inelastic neutron scattering (INS) techniques and polarization analysis. One of the major achievements in this discipline is, in fact, the finding that the magnetic scattering in ferromagnetic metals shows a universal behavior independent of the itinerancy of the magnetic moment. The dynamic susceptibility in the ordered state at low temperatures is well described by spin-wave excitation. In addition, the magnetic scattering function in the paramagnetic phase above the Curie temperature, TC, follows a simple double Lorentzian behavior in both momentum and energy. Then, the energy scale of the spin fluctuations can be expressed in terms of the ratio of the strength of the magnetic correlations defined by the molecular field Curie temperature with respect to TC. The magnetic fluctuations in incommensurate antiferromagnets, i.e., in metallic Cr or in the high temperature superconductors of doped La2CuO4 compounds, show, on the other hand, a rather complicated dynamical structure factor. In contrast to ferromagnetic metals and localized antiferromagnets, the longitudinal fluctuations seem to play an important role deep in the ordered state of Cr.
The spin-wave spectrum in a system with triple-q--> magnetic structure is calculated. The spin waves differ distinctly from those in the corresponding single-q--> structure, but agree with the excitations observed by Lander and Stirling in uranium antimonide (USb). Their experiments thus directly verify that the spins in USb are ordered in the triple-q--> structure.
The adiabatic theory of spin-density waves is developed on the basis of spin-density-functional theory. The wave-number-dependent exchange constant matrix is obtained from spin-density-functional calculations with constrained moment directions. The central assumption considers a fast electronic and a slow magnetic time scale, and postulates negligible correlation of the fast motion between different ionic sites. The parameter-free calculated magnon spectra for Fe, Co, and Ni are in excellent agreement with available experimental data. In the case of Fe, they show strong Kohn anomalies. Using Planck statistics at low temperature, the temperature dependence of the magnetization is well described up to half the Curie temperature. It is conjectured that correlated local-moment clusters survive the Curie transition. On this basis, calculated Curie temperatures are obtained within 10% deviation from experiment for Fe and Co, but 30% to low for Ni.
We examine the spin and charge excitations in antiferromagnetic iron pnictides by mean-field calculations with a random-phase approximation in a five-band itinerant model. The calculated excitation spectra reproduce well spin-wave dispersions observed in inelastic neutron scattering with a realistic magnetic moment for CaFe2As2. A particle-hole gap is found to be crucial to obtain consistent results; we predict the spin wave in LaFeAsO disappears at a lower energy than in CaFe2As2. We analyze that the charge dynamics to make predictions for resonant inelastic x-ray scattering spectra.
We theoretically investigate spin dynamics and -edge resonant inelastic
X-ray scattering (RIXS) of Chromium with commensurate spin-density wave (SDW)
order, based on a multi-band Hubbard model composed of 3d and 4s orbitals.
Obtaining the ground state with the SDW mean-field approximation, we calculate
the dynamical transverse and longitudinal spin susceptibility by using
random-phase approximation. We find that a collective spin-wave excitation seen
in inelastic neutron scattering hardly damps up to 0.6 eV. Above the
energy, the excitation overlaps individual particle-hole excitations as
expected, leading to broad spectral weight. On the other hand, the collective
spin-wave excitation in RIXS spectra has a tendency to be masked by large
spectral weight coming from particle-hole excitations with various orbital
channels. This is in contrast with inelastic neutron scattering, where only
selected diagonal orbital channels contribute to the spectral weight. However,
it may be possible to detect the spin-wave excitation in RIXS experiments in
the future if resolution is high enough.
The study of the magnetic excitations in Mn‐rich alloys should be a good test of the multiband calculations of the spin dynamics of pure γ‐Mn, which is a prototypical itinerant‐electron antiferromagnet. In this paper we report the results of recent neutron inelastic scattering measurements performed on Mn 90 Cu 10 at room temperature (T=0.63T N ) and up to 300 meV of energy transfer. These measurements were performed using the high‐energy transfer (HET) chopper spectrometer at the ISIS spallation neutron source, Rutherford Appleton Laboratory, U.K. The results are in agreement with calculations using a model scattering function convoluted with the full HET resolution function. In this model there is a steep linear spin‐wave dispersion (D≊250 meV Å) for wave vectors up to ≊1/2 of the Brillouin zone and a strong damping which is linear in the wave vector q. For larger wave vectors the spin‐wave energies are constant at about 190 meV. These features are qualitatively similar to the predictions of Cade and Young for pure γ‐Mn.
We report inelastic neutron scattering measurements of the magnetic
excitations in SrFe2As2, the parent of a family of iron-based superconductors.
The data extend throughout the Brillouin zone and up to energies of ~260meV. An
analysis with the local-moment J_1-J2 model implies very different in-plane
nearest-neighbor exchange parameters along the a and b directions, both in
the orthorhombic and tetragonal phases. However, the spectrum calculated from
the J1-J2 model deviates significantly from our data. We show that the
qualitative features that cannot be described by the J1-J2 model are readily
explained by calculations from a 5-band itinerant mean-field model.
Magnetic interactions are generally believed to play a key role in mediating
electron pairing for superconductivity in iron arsenides; yet their character
is only partially understood. Experimentally, the antiferromagnetic (AF)
transition is always preceded by or coincident with a tetragonal to
orthorhombic structural distortion. Although it has been suggested that this
lattice distortion is driven by an electronic nematic phase, where a
spontaneously generated electronic liquid crystal state breaks the C4
rotational symmetry of the paramagnetic state, experimental evidence for
electronic anisotropy has been either in the low-temperature orthorhombic phase
or the tetragonal phase under uniaxial pressure that breaks this symmetry. Here
we use inelastic neutron scattering to demonstrate the presence of a large
in-plane spin anisotropy above TN in the unstressed tetragonal phase of
BaFe2As2. In the low-temperature orthorhombic phase, we find highly anisotropic
spin waves with a large damping along the AF a-axis direction. On warming the
system to the paramagnetic tetragonal phase, the low-energy spin waves evolve
into quasi-elastic excitations, while the anisotropic spin excitations near the
zone boundary persist. These results strongly suggest that the spin nematicity
we find in the tetragonal phase of BaFe2As2 is the source of the electronic and
orbital anisotropy observed above TN by other probes, and has profound
consequences for the physics of these materials.
In a wide variety of materials, such as copper oxides, heavy fermions,
organic salts, and the recently discovered iron pnictides, superconductivity is
found in close proximity to a magnetically ordered state. The character of the
proximate magnetic phase is thus believed to be crucial for understanding the
differences between the various families of unconventional superconductors and
the mechanism of superconductivity. Unlike the AFM order in cuprates, the
nature of the magnetism and of the underlying electronic state in the iron
pnictide superconductors is not well understood. Neither density functional
theory nor models based on atomic physics and superexchange, account for the
small size of the magnetic moment. Many low energy probes such as transport,
STM and ARPES measured strong anisotropy of the electronic states akin to the
nematic order in a liquid crystal, but there is no consensus on its physical
origin, and a three dimensional picture of electronic states and its relations
to the optical conductivity in the magnetic state is lacking. Using a first
principles approach, we obtained the experimentally observed magnetic moment,
optical conductivity, and the anisotropy of the electronic states. The theory
connects ARPES, which measures one particle electronic states, optical
spectroscopy, probing the particle hole excitations of the solid and neutron
scattering which measures the magnetic moment. We predict a manifestation of
the anisotropy in the optical conductivity, and we show that the magnetic phase
arises from the paramagnetic phase by a large gain of the Hund's rule coupling
energy and a smaller loss of kinetic energy, indicating that iron pnictides
represent a new class of compounds where the nature of magnetism is
intermediate between the spin density wave of almost independent particles, and
the antiferromagnetic state of local moments.
A growing list of experiments show orthorhombic electronic anisotropy in the iron pnictides, in some cases at temperatures well above the spin density wave transition. These experiments include neutron scattering, resistivity and magnetoresistance measurements, and a variety of spectroscopies. We explore the idea that these anisotropies stem from a common underlying cause: orbital order manifest in an unequal occupation of and orbitals, arising from the coupled spin-orbital degrees of freedom. We emphasize the distinction between the total orbital occupation (the integrated density of states), where the order parameter may be small, and the orbital polarization near the Fermi level which can be more pronounced. We also discuss light-polarization studies of angle-resolved photoemission, and demonstrate how x-ray absorption linear dichroism may be used as a method to detect an orbital order parameter. Comment: Orig.: 4+ pages; Rev.: 4+ pages with updated content and references
Magnetic excitations in the range 12 THz-30 THz have been measured in chromium at 5 K near the (100) magnetic zone centre. The intensity does not vary appreciably over that range, after corrections for background and resolution effects. The scattering is highly localised in wavevector.
We study the collective magnetic excitations of the recently discovered symmetric spin-density wave states of iron-based superconductors with particular emphasis on their orbital character based on an itinerant multiorbital approach. This is important since the symmetric spin-density wave states exist only at moderate interaction strengths where damping effects from a coupling to the continuum of particle-hole excitations strongly modifies the shape of the excitation spectra compared to predictions based on a local moment picture. We uncover a distinct orbital polarization inherent to magnetic excitations in symmetric states, which provide a route to identify the different commensurate magnetic states appearing in the continuously updated phase diagram of the iron-pnictide family.
The High Resolution Chopper Spectrometer (HRC) was installed at J-PARC, to study dynamics in condensed matters with high-resolutions and relatively-high-energy neutrons. Although the HRC was damaged by the earthquake disaster, we recovered and started neutron scattering experiments. During the recovery works, we also improved the performance of the HRC, by installing a supermirror guide tube, a collimator system, an experiment control environment etc.
Most iron-based superconductors undergo a transition to a magnetically
ordered state characterized by staggered stripes of parallel spins. With
ordering vectors or , this magnetic state breaks the
high-temperature tetragonal symmetry of the system, which is manifested by a
splitting of the lattice Bragg peaks. Remarkably, recent experiments in
hole-doped iron arsenides reported an ordered state that displays magnetic
Bragg peaks at and but remains tetragonal. Despite being
inconsistent with a magnetic stripe configuration, this unusual magnetic phase
can be described in terms of a double- magnetic structure
consisting of an equal-weight superposition of the ordering vectors
and . Here we show that a non-collinear double- magnetic
configuration, dubbed \emph{orthomagnetic}, arises naturally within an
itinerant three-band microscopic model for the iron pnictides. In particular,
we find that strong deviations from perfect nesting and residual interactions
between the electron pockets favor the orthomagnetic over the stripe magnetic
state. Using an effective low-energy model, we also calculate the spin-wave
spectrum of the orthomagnetic state. In contrast to the stripe state, there are
three Goldstone modes, manifested in all diagonal and one off-diagonal
component of the spin-spin correlation function. The total magnetic structure
factor displays two anisotropic spin-wave branches emerging from both
and momenta, in contrast to the case of domains of stripe order,
where only one spin-wave branch emerges from each momentum. We propose that
these unique features of the orthomagnetic state can be used to unambiguously
distinguish it from the stripe state via neutron scattering experiments, and
discuss the implications of its existence to the nature of the magnetism of the
iron arsenides.
The itinerantly antiferromagnetic or spin-density-wave state is examined for an electron gas and a more realistic band model. The paramagnetic electron gas, which is unstable to the formation of an antiferromagnetic state in the Hartree-Fock approximation, is shown to be stable when either static Fermi-Thomas or dynamic screening is included. A mathematical model is constructed for the spin-density-wave state below its transition temperature and its experimental consequences examined. A criterion for itinerant antiferromagnetism in a band model is derived and shown to be consistent with the antiferromagnetism of Cr. A crude model is employed to compute qualitatively the thermodynamic and electromagnetic properties of an itinerant antiferromagnet like Cr. In particular, the specific heat, spin susceptibility, collective modes, and electromagnetic absorption are evaluated.
Spin waves in the antiferromagnetic alloy γ-Fe0.5Mn0.5 have been studied at by the inelastic neutron scattering technique. We observed an isotropic dispersion and obtained a value for the spin-wave velocity of 255 ± 30 meV Å (3.88 ± 0.50 × 106 cm/sec), which is the order of the spin-wave velocity in Cr (a typical itinerant antiferromagnet). The energy gap at q = 0 was found to be 7.0 ± 0.5 meV. These results suggest the existence of a long-range spin ordering in the conduction electrons of this alloy.RésuméEs wurden Spinwellen in der antiferromagnetischen Legierung γ-Fe0.5Mn0.5 bei mit inelastischer Neutronenstreuung untersucht. Wir fanden isotrope Dispersion und einen Wert 255 ± 30 meV Å (3.88 ± 0.50 × 106 cm/sec) für die Spinwellen geschwindigkeit, der von der Gröβenordnung der Spinwellengeschwindigkeit in Cr ist (ein typischer Band-Antiferromagnet). Die Energielücke bei q = 0 ergab sich zu 7.0 ± 0.5 meV. Diese Resultate legen die Existenz einer langreichweitigen Spinordnung der Leitungselektronen in dieser Legierung nahe.
We have theoretically shown for the first time that the multi spin density wave (MSDW) state is the most stable among other
spin density wave states in γ-FeMn alloys. Use is made of an extended two-band model to describe various spin density wave
states. Furthermore we study theoretically the spin fluctuation in the MSDW state near the Néel temperature. The present theory
predicts the existence of three distinct spin fluctuations; spin wave mode, spin distortion mode and damping diffusion mode.
The first two modes possess a phonon-like dispersion except for nonvanishing energy gap.
The magnetic critical scattering from a single crystal of gamma-Fe0.537Mn0.463 has been studied around the (001) and (110) reciprocal lattice points. It has been found that the longitudinal spin correlation becomes divergent at the Néel temperature TN of 479.0± 1.5 K, the parallel staggered susceptibility chi//(Q) obeys a power law of alpha(T-TN)-1.35± 0.05, in contrast with the temperature independent static susceptibility. The transverse spin correlation, however, remains finite at TN with the inverse correlation length of about 0.038 Å-1. The anisotropy of the spin correlation has been suggested not due to the crystal anisotropy, but to either the anisotropic exchange interaction or the anisotropy of the non-interacting spin susceptibility of d bands. The anisotropy of the scattering profile with respect to the wave vector around the reciprocal lattice points has been found much smaller than that expected for the type I antiferromagnet with the short range interactions.
The antiferromagnetism of disordered gamma-FeMn alloys has been studied by neutron diffraction and Mössbauer effect. The average magnetic moment per atom at 0°K decreases with decreasing Fe concentration from 1.94muB for 69.1 at% Fe to 1.08muB for 47 at% Fe. The hyperfine field at 0°K acting on the Fe nucleus, however, remains almost constant at about 40 kOe±3 kOe for this concentration range. The temperature dependence of the sublattice moment for 69.1 at% Fe fits with the Brillouin function BJ(x) with J{=}1. The decrease of the moment near the Néel temperature becomes steeper when the concentration of Fe is lower. In contrast with it, the temperature dependence of the hyperfine field is independent of concentration, and is markedly different from the temperature dependence of the sublattice moment for any concentration. The results indicate that the magnetic moments on Fe and Mn atoms behave with temperature in fairly different ways in this alloy system. The magnetic form factor of gamma-FeMn alloys was also determined.
The antiferromagnetism of polycrystalline and single crystal samples of disordered γ Fe-Mn alloys containing between 20 to 50 at% Mn was investigated by means of a magnetic balance, a sensitive torque magnetometer and neutron diffraction. The alloys with 20 to 27 at% Mn exhibited ε↔γ transformation in the same temperature range as the magnetic transformation. The relation between the magnetic and the crystallographic transformations was investigated and a phase diagram of Fe-Mn alloys in this composition region was obtained. A neutron diffraction study of single crystal samples confirmed the generalized antiferromagnetic spin structure proposed by the powder neutron diffraction study of Kouvel and Kasper. The torque measurements, made on single crystals which had been cooled through the Néel point in a strong magnetic field, indicated that the spin structure is intrinsically cubic, the spins on the four sublattices being directed toward the different cube diagonals. The results are discussed from the viewpoint of a molecular field theory based on the localized electron model. However, the temperature dependence of the susceptibility below and above the Néel point cannot be interpreted on the basis of a localized electron model, although the latter can account for the proposed spin structure.
To study condensed-matter dynamics over a wide energy–momentum space with high resolution, we constructed the High Resolution Chopper Spectrometer (HRC) at beamline BL12 at the Materials and Life Science Experimental Facility (MLF) of the Japan Proton Accelerator Research Complex (J-PARC). With the construction nearly completed, we proceeded to characterize the HRC. We confirmed that, under limited conditions, the neutron intensity and energy resolution of the HRC agree well with the design values.
Inelastic neutron scattering experiments were carried out onthe spin density wave (SDW) stateof a single- Q chromium crystal. We made systematic investigationsin a wide energy range up to omega˜100 meV by combiningthe triple axis and time-of-flight neutron spectroscopy.The q profile of the inelastic scattering is of a three-peakedstructure; the commensurate scattering (CMS)at the antiferromagnetic superlattice and the incommensurate scattering (ICMS)near the wave vector points of the static SDW. In the longitudinal SDW statethe dynamical susceptibility for the CMS increases with increasing omegaand eventually overwhelms that for the ICMS. The ICMS, on the other hand,reaches a maximum at omega˜20 meV. This cross-over phenomenonis reflected in the temperature dependence of the magnetic excitations so thatthe CMS dominates at temperatures above 200 K.
By using a real space expansion approach to calculate the Green function of d-electrons, the four-spin interaction is derived and the relative stability among various multiple spin density wave states is discussed for antiferromagnetic fcc transition metals with the first-kind ordering. The transition from a triple-Q state to a single-Q state is shown to occur with increasing or decreasing the number of d-electrons from the nearly half-filled band region.
Spin waves in an antiferromagnetic ordered alloy FePt3 have been investigated in a wide range of temperature by means of neutron inelastic scattering techniques. From the dispersion relations of the spin waves at 5 K, the exchange interaction parameters between Fe atoms were determined based on the localized spin model, which were found to extend in long range with an oscillatory character. However, a renormalized spin wave theory with these parameters fails to reproduce the observed temperature dependence of the sublattice magnetization as well as the renormalization of the spin wave energy. This fact, together with the relatively large damping of the spin waves, suggests that the simple localized spin model is not sufficient to describe the spin dynamics in the alloy.
The spin wave excitations in the itinerant antiferromagnetics of gammaFeMn alloys have been studied by neutron scattering. The isotropic dispersion relation has been observed for gammaFe0.7Mn0.3 and gammaFe0.5Mn0.5. The velocities of spin waves at room temperature are 230± 30 meVA, 220± 30 meVA and 160± 30 meVA for Fe0.7Mn0.3, Fe0.5Mn0.5 and Fe0.3Mn0.7, respectively. All samples have the spin wave energy gaps of about 10 meV at 0 K which shows a similar temperature dependence. The damping of the spin wave is significant and it depends on the momentum transfer of the spin waves. The magnon-like excitation persists even 200 degrees above the Néel temperature for Fe0.5Mn0.5. The results are favorable to the itinerant electron model where the multiple spin density wave state is assumed to exist.
The specific heat and the thermoelectric power of gamma-FeMn alloys were measured for three different compositions. The peak of the specific heat at the Néel temperature is broad for Fe70Mn30 but it becomes sharp when the concentration of Fe decreases. This tendency is interpreted in terms of the temperature variation of the sublattice magnetization of the alloys. The magnetic entropies obtained are 1.4, 0.83 and 0.57 cal/mol. deg. for Fe70Mn30, Fe60Mn40 and Fe50Mn50 respectively. These values are nearly a half of those estimated for the localized spin system. The thermoelectric power is also strongly composition dependent. The temperature dependence of the thermoelectric power suggests that two types of carriers exists in the alloys. Only a small anomaly was observed at the Néel temperature. The results suggest that the antiferromagnetism of gamma-FeMn alloys is distinctly different from that of Cr.
A comprehensive account is given of the macroscopic and microscopic physical properties of chromium (and where appropriate those of its dilute alloys) that relate to its antiferromagnetism. Neutron scattering is treated in great detail, first in the historical introduction, then as an experimental probe of both the magnetic structure and the excitations of the incommensurate spin-density-wave state and (with the assistance of x rays) of the concomitant charge-density wave and strain wave. Neutron scattering is considered as a tool to explore not only the disappearance of long-range order with increasing temperature through the growth of excitations as the weak first-order Néel transition is approached, but also the persistence of these spin fluctuations well into the paramagnetic state-processes that are still little understood. The article surveys, without mathematical details, model systems designed to reproduce the magnetic and thermodynamic properties of Cr. The energy-band structure calculations are given a more comprehensive review. Special attention is paid to calculations of the wave-vector-dependent susceptibility that reproduce the observed wave vector of the spin-density wave, and to a recent finite-temperature calculation that gives almost the right Néel temperature. The review of Fermi-surface studies emphasizes those designed to relate the spin-density wave vector (and its pressure dependence) to the nesting vector of the Fermi surface. An account is given of the spectroscopic determination of the energy gap(s), whose theoretical analysis is still unclear, and of experiments aimed at determining physical properties that throw light on the origin of the weak first-order Néel transition. The article describes the use of magnetic anomalies in the elastic moduli to determine the volume dependence of the exchange interaction responsible for antiferromagnetism in Cr. The experimental features of the spin-flip transition are reviewed, although a theory of this phenomenon is wanting. The experimental study of microscopic structure by the use of hyperfine-interaction properties is surveyed. An account is given of both experimental and theoretical studies of the surface of Cr and of Cr films and sandwiches. Finally, "technical antiferromagnetism" is discussed: the effect of severe internal strain in producing a commensurate antiferromagnetic state, wave-vector Q domains, polarization S domains (for which the experimental evidence is scanty), and ultrasonic attenuation as a tool to study them.
Neutron diffraction measurements at 77 and 298 °K reveal long-range antiferromagnetic ordering in the atomically-disordered face-centered cubic alloys with more than about 50 at. per cent Fe in the ternary series (Ni,Fe)3Mn. Consistent with these results, the Néel temperatures of these alloys, deduced from the temperature dependence of their electrical resistivities, rise from 210 to 425°K as the iron concentration increases from 50 to 75 at. per cent. An antiferromagnetic structure that conforms to the neutron diffraction polycrystal data is the collinear, tetragonal configuration previously reported for the Mn-rich Mn-Cu alloys. However, quite unlike Mn-Cu, the Fe-Ni-Mn alloys give no detectable X-ray diffraction evidence for a tetragonal (or any other) distortion of their cubic atomic arrangement. Hence, an alternative structure proposed for these alloys is one that is cubic though not collinear, the moments on four equivalent sublattices being oriented along different 〈111〉 directions such that their vector sum is zero. Both these magnetic structures are then shown to be special cases of a general set of non-collinear, tetragonal configurations which produce the same neutron diffraction pattern for a given magnitude of the average atomic moment gm For the (Ni,Fe)3Mn series it is found that gm increases from 1.1 to 1.7μB with increasing iron concentration from 50 to 75 at. per cent, which is shown to be at least in part due to an improved degree of long-range magnetic order. The results of the present work are compared and related to the unusual ferromagnetic-antiferromagnetic behavior of the more Ni-rich alloys in this ternary series and to the low-temperature antiferromagnetic structure recently reported for γ-iron.
We have performed neutron inelastic scattering experiments in order to investigate the high-energy magnetic excitations in chromium. The commensurate diffuse mode has been measured up to an energy of 550 meV, the mode remains highly localised in Q and persists past an expected energy gap at 440 meV.
The first-principles calculations for FeMn and MnPt alloys both of ordered and disordered phases have been performed with the TB-LMTO method combined with the CPA. The magnetic structure of FeMn in the disordered phase is suggested to be non-collinear 3Q structure where the average moment is around 1.7 µ B and the Neel temperature estimated from the effective exchange constant is about 550 K. For MnPt system, the high stability of the ordered phase may be closely related to the pseudo-gap formed at the Fermi level, which is brought about by the collinear antiferromagnetic (AF) staggered field. Accordingly, the calculated Neel temperature is quite high (˜760 K) as much as the measured value (˜1000 K). In the disordered phase, on the other hand, the random distribution eliminates the pseudo-gap and the magnetic structure turns likely to be 3Q structure with the Neel temperature down to around the room temperature.
The band structure of both paramagnetic and antiferromagnetic gammaMn is evaluated by G.F.M. with the Slater exchange approximation. The observed magnetic moment (2.3muB) is self-consistently obtainable, if the full Slater exchange is reduced by about one-half. The situation is the same at Cr. No special charactors are observed in E(\mbi{k})-curves of gammaMn contrary to the case of Cr. Introducing a simple exchange interaction term -Jeffdelta n2 by the random phase approximation, we extend Mueller's interpolation scheme to apply to the antiferromegnetic case, and antiferromagnetism in f.c.c. structure is examined within the limit of the rigid band approximation. If we assume the value Jeff{=}0.060Ry, f.c.c. Mn and f.c.c. Fe are antiferromagnetic and their magnetic moments are determined as 2.3muB and 0.7muB, respectively, which are in good agreement with experimental values. In f.c.c. structure the gap mechanism similar to Cr is found to be effective around N≈7.3. The theoretical prediction seems to be supported by experiments of gammaFeMn alloy.
The magnetic properties of fcc-FeMn alloys, especially at the Fe0.5Mn0.5 composition, have been the subject of intense experimental and theoretical investigations for several decades. We carry out an ab initio theoretical study of this system, including simultaneous optimization of structural and magnetic properties, and find that the ground state is the locally relaxed noncollinear 3Q antiferromagnetic structure. We also show that the two most frequently used parameterizations of the generalized gradient approximation not only fail to reproduce the equilibrium lattice constant of FeMn alloys, and consequently the magnetic properties, but also internally yield qualitatively different results. For practical studies of these alloys, which currently attract great attention, we propose a set of approximations, which is internally consistent, and brings the equilibrium lattice constant and magnetic properties in good agreement with the experiment in the whole range of alloy compositions.
The spin excitations in a single crystal of Cr0.98Mn0.02 have been studied by inelastic neutron scattering. High-resolution triple-axis spectrometer measurements have yielded a value of the spin-wave velocity at T/TN≃0.5 of (1.30 ± 0.15) × 107 cm/sec in good agreement with earlier measurements. The excitation strength drops extremely rapidly in the vicinity of TN and decreases approximately linearly above TN. The spin-wave form factor has been measured below and above TN by measuring the spin-wave intensity at various superlattice points, and is found to agree to within experimental error with the static spin form factor. The implications of these results for the theory of itinerant electron antiferromagnetism are discussed.
Magnetic excitations in an intermetallic compound MnSi have been studied by neutron scattering. At 5 K in a magnetic field of 10 kOe, well-defined spin-wave excitations have been observed below 2.5 meV. The dispersion relation is almost isotropic and is hωq(meV)=0.13+52q2 (Å-2) in the [100] direction. Above 3 meV, the excitation linewidth increases substantially, suggesting that the dispersion relation merges into the Stoner continuum. The Stoner excitations, which extend over almost all of the Brillouin zone, show a broad peak on the extension of the spin-wave dispersion relation. The spin-wave excitation renormalizes with increasing temperature and collapses into critical scattering above 40 K. On the other hand, the excitation in the Stoner continuum is affected little by temperature; the excitations are qualitatively the same at T/TN=10 as at 5 K. The Stoner boundary energy decreases with increasing temperature. The results provide us with the first example of magnetic excitations in a weak itinerant ferromagnet.
That metallic antiferromagnetism can be verified by employing the Mössbauer effect has been demonstrated for fcc FeMn alloys near the compositions of equal atomic fractions and for a hcp Fe‐Mn alloy with 17% Mn. The hyperfine fields deduced from measurements of line broadening of incompletely resolved spectra give 33, 39, 43, 42, and 22 kG for 60, 50, 45, and 43% Mn in the fcc phase and 17% Mn in the hcp phase, respectively. The uncertainty is estimated at ±2.5 kG. Néel temperatures of 500°K are found for the four fcc alloys. The hcp alloy has a Néel temperature of 240°K. These values agree with resistivity and susceptibility data. In the light of this treatment of incompletely resolved spectra, a suggested reinterpretation of previous data on antiferromagnetic Cr is offered.
The magnetic phase diagram of γ Fe-Mn alloys has been investigated
in the whole range of composition by neutron diffraction and other
techniques. The f.c.c. phase of Fe-Mn alloys with less than 40 at. %
iron and more than 85 at. % iron was stabilized by addition of small
amounts of copper and carbon respectively in the alloys. An
antiferromagnetic long range order was detected in the whole range
composition. The antiferromagnetism of this alloy system
Fex-Mn1-x was found to be classified into three
distinct groups according to the composition. They are 1) 0≤x≤0.3,
2) 0.4<x<0.8, and 3) 0.8≤x≤1. The magnetic phase of the
second group is markedly different from other two groups. The results
agree qualitatively with the prediction based on the band theory.
A method of calculating the long wavelength spin wave frequencies in a multiple band itinerant antiferromagnet is described. No special features of the band structure, such as nesting, are assumed. The frequencies are derived by examining the poles of the RPA generalized susceptibility for the d electrons. A natural consequence of the treatment is the existence of a spin wave damping proportional to the wavevector. This is attributed to the decay of the spin wave into an electron-hole pair. The formulae are applied numerically to gamma manganese. Rough agreement with experiment is found.
Mossbauer transmission spectra taken, at a temperature of 4.2 K, through a thin single-crystal sample gamma -Fe-Mn, in magnetic fields up to 9 T are presented. These spectra have been fitted to the three energetically favoured magnetic spin models of gamma -Mn. The authors' results are not consistent with the (001) model of a collinear antiferromagnet, whereas fits to the (111) and (110) models are good but indistinguishable.
The method of anisotropy measurement of gamma -ray emission from spin-polarised nuclei has been applied to the FCC Fe54Mn46 alloy to deduce the ground-state magnetic structure. Measurements were made at temperatures down to 240 mu K, and the observed anisotropy of the emission from 54Mn isotopes embedded in the single-crystal sample gives definitive evidence for the triple-Q spin-density wave in this material.
For pt.I see ibid., vol.17, p.1419 (1987). Spin-density measurements for a single crystal of the FCC alloy Fe66Mn34 have been performed using elastic and inelastic neutron scattering. Combining the results from elastic scattering with the experimentally determined symmetry of the spin wave spectrum it has been unequivocally deduced that this alloy has a collinear magnetic structure. The magnetic form factor of the alloy, the average magnetic moment per atom and the spin-density asphericity have been obtained from a Fourier inversion of the neutron diffraction data. Combining these results with a measurement of the magnetic scattering amplitude of a powdered sample of the alloy Fe82Mn13C5 allows the magnetic moments of Fe and Mn in the iron-rich region of the Fe-Mn system to be discussed. Comparison of the experimental charge- and spin-density asphericities of Fe66Mn34 with available calculations of the band structure of FCC Fe shows relatively poor agreement. The possibility of modifying the band structure of FCC Fe by introducing band splittings corresponding to a state-dependent potential is discussed and shown to lead to better agreement with experimental data.
The paper summarizes the results of neutron scattering from a weak itinerant ferromagnet MnSi (the itinerant electron system) and Invar alloys: Fe 65 Ni 35 and Fe 3 Pt (the quasi‐localized spin system). Well defined spin wave excitations as well as the excitations in the Stoner continuum could be detected at 5 K for MnSi in the induced ferromagnetic state. The spin dynamics in this weak itinerant ferromagnet were found to be compatible with a simple theory of an itinerant ferromagnet within RPA. The Stoner boundary is located at about 2.5 meV and the magnetic properties of MnSi are mainly determined by the Stoner excitations. In the case of Invar alloys, spin wave excitations could be detected up to 80 meV. The itinerant character of the magnetic properties is suggested not to be due directly to the Stoner excitations but to interactions between the spin waves and the Stoner modes in some higher order mechanisms.
Chromium and its dilute alloys are unique examples of magnetism caused by itinerant electrons. The magnetic excitations have been studied by inelastic neutron scattering using a high‐resolution triple‐axis spectrometer. Spin‐wave peaks in q scans at constant energy transfer ℏω could, in general, not be clearly resolved at any temperature below T N but it is still possible to deduce the slope ω/q of the dispersion curve and also to estimate the spin‐wave lifetimes. The scattering displays a divergence as q→0, ω→0, T→T N characteristic of critical fluctuations. The critical scattering is confined to small values of q, but the ω range is very wide compared to critical scattering from systems with localized magnetic moments. The experimental results agree generally with the theoretical predictions of Liu [Phys. Rev. Lett. 23, 311 (1969); Phys. Rev. (to be published)].
For strongly ferromagnetic or antiferromagnetic alloys, as well as nonmagnetic alloys, the γ values determined by low temperature specific heat measurements correlate quite well with the electron concentration. These values may be considered as representative of the electronic specific heat of the alloys concerned. From them the approximate shape of the d-band was determined for f.c.c. alloys in the electron concentration range 7 to 10 e/a. However, for alloys known from magnetic measurements to have a complex magnetic structure (such as long range ferromagnetism with superimposed short range antiferromagnetism), γ does not correlate with the electron concentration. As shown for disordered MnNi3 by specific heat measurements after magnetic cooling to 1·4°K, in such cases γ comprises, in addition to the electronic specific heat, a magnetic contribution, which is in some cases very large.
We study the magnetic excitation spectra in the paramagnetic state of BaFe(2)As(2) from the ab initio perspective. Both the one-particle and the magnetic two-particle excitation spectra are determined within the combination of the density functional theory and the dynamical mean-field theory method. This method reproduces all the experimentally observed features in inelastic neutron scattering and relates them to both the one-particle excitations and the collective modes. The magnetic excitation dispersion is well accounted for by our theoretical calculation in the paramagnetic state without any broken symmetry; hence, nematic order is not needed to explain the inelastic neutron scattering experimental data.
We argue that salient experimental features of the magnetic excitations in the spin-density-wave phase of iron-based superconductors can be understood within an itinerant model. We identify a minimal model and use a multiband random-phase approximation treatment of the dynamical spin susceptibility. Weakly damped spin waves are found near the ordering momentum and it is shown how they dissolve into the particle-hole continuum. We show that ellipticity of the electron bands accounts for the anisotropy of the spin waves along different crystallographic directions and the spectral gap at the momentum conjugated to the ordering one. We argue that our theory agrees well with the existing neutron scattering data.